The present invention relates to gas-permeable thermal insulators, and more particularly, to gas-permeable thermal insulators for use in heating ovens in which objects are heated directly or indirectly with heating gas and from which exhaust gas should be vented.
In the specification, "heating gas" means gas which is used to heat directly or indirectly an object to be heated in a heating oven and "exhaust gas" means gas which has heated the object and is to be exhausted from the heating oven. In other words, "exhaust gas" is heating gas which has finished heating the object.
Most of heating ovens are of the type in which fuel gas or oil is burned to provide hot combustion gases to heat an object to be heated therewith. In the prior art such a heating oven was thermally insulated by attaching to the wall thereof a thermal insulator in the form of a foam having a closed cellular structure such as rigid polyurethane foam, thereby achieving energy saving. However, little attention was paid to waste heat entrained with the exhaust gas except that heat exchange was conducted out of the heating oven to make effective use of the waste heat in some cases. It has not been practiced to directly reuse waste heat in heating an object to be heated within an oven as by thermally insulating an exhaust vent.
Generally, in the heat treatment of certain parts in an oven using hot gases resulting from the combustion of fuel gas or oil, the parts may be heated directly with the hot combustion gas by bringing the hot combustion gases in direct contact with the parts. In another case, the parts may be heated indirectly with the hot combustion gas by introducing the hot combustion gases into a radiant tube which is provided in an oven and emits radiations to heat the parts. In both cases, the reuse of waste heat entrained with the exhaust gas has not been fully taken into account.